According to Advertising Age, the appearance has Audi convinced that the books have contributed to its growing sales - but the carmaker says it didn't pay a single penny for the mention.

Speaking with the paper, a representative for 50 Shades of Grey publisher Random House said that no deal was made, and author E. L. James does not have any special love for Audi cars.

"She does not drive an Audi. She drives a MINI and her husband drives a Volkswagen Touareg," Random House' Russell Perreault said.

Audi of America's Loren Angelo told Advertising Age that while the brand has no data that proves any connection between its improving sales and the 50 Shades series, the placement was a welcome surprise.

"We're seeing our products utilized more and more within popular culture," Angelo told the paper. "Those organic results are always a sign that a brand is generating real cultural relevance."

"Obviously, the runaway success of this series has introduced Audi to consumers who might not have previously been thinking of the brand and that certainly has a positive impact for us."

Audi isn't the only brand of choice for the series' characters, however, with both the Saab 9-3 and Mercedes-Benz CLK cars getting a guernsey - and neither car can still be bought brand-new.